The present invention relates to an electrophotographic recording apparatus, and more particularly to a method and apparatus for controlling exposure and transfer units in an electrophotographic recording apparatus capable of eliminating unnecessary exposure and reverse charge of a photosensitive drum by controlling the exposure in response to an actually supplied paper size.
Generally, a printer of an electrophotographic recording apparatus has, as shown in FIG. 1, a photosensitive drum 107 for forming an electrostatic latent image, a charge unit 104 for providing a uniform negative charge to the photosensitive drum 107, an exposing unit 105 for generating the electrostatic latent image to the photosensitive drum 107, a developing unit 108 for developing a latent image exposed to the photosensitive drum 107, a transfer unit 106 for transferring a developer formed in the photosensitive drum 107 to a print medium such as paper, a register roller 109 for feeding the paper, a feed path 98 upon which the paper is fed through the printer a fusing unit 103 for fusing the developer transferred to the paper, and resister sensors 101 and 111 for sensing the feeding state of the paper.
Referring to FIG. 2, a block diagram of a controller for controlling the printer of FIG. 1 is shown. A mechanical controlling circuit 201 produces signals for controlling a motor and each mechanical part. Control signals for controlling the charge, developing and transfer units 104, 108 and 106 are generated from a developing processor controlling circuit 202. A sensor input circuit 203 receives the sensed values of various sensors, and an image data generating circuit 205 generates data corresponding to contents to be printed on the paper. An interface circuit 206 is interfaced with an external unit, and a central processing unit (CPU) and controlling circuit 204 controls the whole controller.
In the case of printing, the system constructed as shown in FIGS. 1 and 2 waits for a printing command from a ready state. When the printing command is given, the paper size to be printed and conditions of a developing processor are set in the interior of the CPU and controlling circuit 204. A paper feeding condition of the developing processor is set, after arranging the paper in the register roller 109, by subtracting the exposing length AB on the photosensitive drum 107 from the length BD between the transfer unit 106 and the register roller 109. After feeding the paper by the set length (B.sub.b -A.sub.B), the exposure is started to equalize a top margin of the paper and data. In this case, if the paper is normally fed, the paper corresponding to the set length is exposed to the light and the exposing and developing conditions are released. Thereafter, a printing operation is ended by discharging the paper and the system waits for a next command. If there is an error in the paper feeding, the error is sensed by the jam sensors 101 and 111 wherein the sensor 111 is a register sensor and the other sensor 101 is an exit sensor, and processed by the CPU and controlling circuit 204.
Meanwhile, the CPU and controlling circuit 204 receives and processes the data through the interface circuit 206 from a computer or a video controller and transmits it to the image data generating circuit 205. The image data generating circuit 205 processes the data and provides it to the exposing unit 105.
In more detail, as shown in FIG. 3, if the printing command is given, the paper size is determined, at step 3a, by a cassette sensor. The paper is picked up from a cassette and paper feeding is started at step 3b. At steps 3c and 3d, the charge and exposing units 104 and 105 are turned on. The CPU and controlling circuit 204 checks, at step 3e, whether or not the top of the paper reaches the transfer unit 106 through the register roller 109. If the top of the paper reaches the transfer unit 106, at step 3f, the CPU and controlling unit 204 turns on the transfer unit 106 to transfer a toner image formed on the photosensitive drum 107 to the paper. At step 3g, a check is made to see if the bottom of the paper reaches the transfer unit 106. If the bottom of the paper reaches the transfer unit 106, the transfer unit 106 is turned off at step 3h. At step 3i, the fusing is performed, and at step 3j, the charge unit 104 is turned off. That is, the transfer unit 106 is turned off after the paper, A4 size for example, passes as much as the A4 size.
As described above, in the case of printing a shorter paper than a standard paper of the cassette, since the transfer unit 106 is turned on until the time of standard paper passes out, the toner is dropping to the transfer unit 106. Consequently, a printing image of a next paper is affected. Further, since the photosensitive drum 107 is reversely charged by a positive voltage of transfer unit, the printing quality degrades. In other words, if a shorter paper than a standard paper is fed, data is generated on the part where the paper does not exist. At this time, the photosensitive drum 107 is subjected to exposure by the exposing unit 105. Thus, since the developer is transmitted to the photosensitive drum 107 from the developing unit 108, the transfer unit 106 is contaminated and the transfer efficiency is reduced. Moreover, the developer transferred to the transfer unit 106 is unnecessarily consumed and the photosensitive drum 107 is unnecessarily exposed, thereby shortening the printing machine's life.